This invention relates to an electromechanical filter, and more particularly, to an electromechanical filter comprising an external matching circuit at each of the input and output ends.
An electromechanical filter comprises a mechanical resonator and a pair of electromechanical transducers mechanically coupled to each other with the resonator interposed. When the filter further comprises an additional mechanical resonator or resonators, all resonators are sequentially interposed between the transducers. As taught in a paper contributed to Proc. 20th Electronic Component Conference (May 1970) by Morio Onoe and Takeshi Yano, the latter being one of the present applicants, the filter may comprise an additional electromechanical transducer or transducers.
Although characterized by compactness and steep and stable filter characteristics, an electromechanical filter is defective in that undulations, usually called "ripples", grow intolerably large in the filter characteristics when a change occurs in the ambient temperature beyond a certain limit. The growth of ripples results from the fact that terminal impedances do not follow changes caused by the temperature change in equivalent inductances of the respective transducers and also from the fact that each of their resonant frequencies takes a value different from a predetermined resonant frequency due to the above temperature change. A change in the transducer resonant frequency is suppressed by achieving in each of the transducers a counterbalance between temperature coefficients of Young's moduli of a piezoelectric ceramic piece and an elastically invariant alloy piece made of iron, nickel, chromium and titanium attached to one or each of the ceramic piece faces. The temperature change of the equivalent inductances depends mostly on the temperature coefficient of dielectric constant of the piezoelectric ceramic material.
Although attempts have been made to reduce the temperature dependency by minimizing the temperature coefficient of the dielectric constant, the results have indicated that the minimum dependency achieved amounts to about .+-. 10% for a temperature change of 40.degree. C and that a limit is inevitably imposed on improvement of the temperature characteristics of an electromechanical filter. Another approach to the temperature characteristics improvement is disclosed in an article contributed by Toshio Ashida to a Japanese technical periodical, Densi Tusin Gakkai Ronbunsyu (The Transactions of the Institute of Electronics and Communication Engineers of Japan), Vol. 57-A, pp. 345-352 (No. 5) entitled "Ondo-tokusei o koryosita kikai-hwiruta-yo Atuden-henkansi no Sekkei" or "Design of Piezoelectric Transducers for a Temperature-Stable Mechanical Filter". In the Ashida article, relations are derived between the temperature dependency of filter characteristics and changes in the parallel capacitance, capacitance ratio, and resonant frequency of the electromechanical transducer in both cases where the filter is accompanied and not accompanied by a pair of external coils. The article thus provides an electromechanical filter wherein a counterbalance is reached between the temperature dependency and the temperature changes. On the other hand, it is conventional to make an electromechanical filter comprise a pair of external matching circuits, each comprising a coil as in the Ashida article and a temperature-compensation capacitor. In any event, a temperature range in which an electromechanical filter is operable with a one-twentieth C.C.I.T.T. standard (one-twentieth of the characteristics defined in International Telephone and Telegraph Consultive Committee Recommendation G-132) satisfied has been very narrow, being restricted to 4.degree. C-43.degree. C. Incidentally, the matching circuits are called external because they, although included in an electromechanical filter in a sense, are external to that combination of the transducers and resonator or resonators which serves by itself as an electromechanical filter.